Fluid shifts between peripheral tissues and the blood stream are studied in a) man immersed in water, and b) bluefish resting on a V board in air, and tilted head up at 30 degrees. A general hypothesis is being evolved, indicating that locomotion in water led to the evolution of body structure capable of withstanding hydrodynamic forces during swimming of fish. This, in turn, allowed land animals to withstand gravity without having the blood and fluid displaced downward excessively. BIBLIOGRAPHIC REFERENCES: DuBois, Arthur B., Cavagna, Giovanni A. and Fox, Richard S. Locomotion of Bluefish. The Journal of Experimental Zoology, Vol. 195, No. 2, February 1976. A.B. DuBois, P. Carniol, and R.D.B. Morris. Effect of Gravity and Exercise on the Heart Rate and Blood Pressure of Bluefish (Pomotamus saltatrix). (abstract) The Biological Bulletin, Vol. 149, No. 2, p. 425, October 1975.